Removal of the acid gases carbon dioxide and/or hydrogen sulfide from natural gas, petroleum, hydrogen, and coal gas is important from an environmental standpoint since such gases are highly toxic and corrosive, often contributing to the phenomenon known as "acid rain." Such acid gases are also quite destructive to methanation catalysts. Generally, removal is accomplished by a number of scrubbing processes utilizing absorbents or solvents which reversibly absorb the acid gas. For example, the Benfield and Catacarb processes utilize activated carbonate absorbents, the monoethanolamine and diglycolamine processes use aqueous amine solutions, while the Purisol and Sulfinol processes use simple physical solvents. In the case of the Purisol process, the solvent is N-methyl-2-pyrrolidone, and raw gas is contacted with a countercurrent flow of the absorbing solvent, the solvent thereafter being regenerated by flashing and stripping.
However, all such conventional scrubbing processes are quite costly in terms of capital and operating expense since they require absorption in large-volume, high pressure towers, desorption in low pressure generators or stripping columns, extensive pumping for solvent recirculation, and the generation of substantial amounts of steam for stripping. It is estimated that nearly a third of the cost of producing gaseous fuels such as hydrogen and methane from coal is attributable to coal gas cleanup by such processes.
Removal of hydrogen sulfide from coal gas with an immobilized liquid membrane comprising carbonate solution supported in discrete pores of an unspecified microporous hydrophilic polymer membrane was made by Matson et al. and reported in 16 Ind. Eng. Chem. Proc. Des. Dev., 370 (1977), the extent of the removal being limited to 15 to 30%. Hydrogen sulfide was also removed from a mixture of hydrogen sulfide and nitrogen by Heyd et al. and reported in 2 J. Memb. Sci. 375 (1977), the removal being effected by unsupported vinylidene fluoride polymeric membranes modified by the addition of 10% by weight of various amines and other agents. Although the use of 1-methyl-2-pyrrolidone is disclosed as one of the modifiers, the results obtained were less satisfactory than with an unmodified membrane.
In the production of synthetic natural gas from coal gas (comprising stem, hydrogen, carbon monoxide, carbon dioxide, methane and small amounts of hydrogen sulfide), the concentration of methane is increased in a series of steps which involve the removal of carbon dioxide and hydrogen sulfide since carbon dioxide interferes with the shift conversion reaction step and hydrogen sulfide tends to poison methanation catalysts. In the series of methane-enrichment steps, hydrogen and carbon monoxide are desirably left in the process gas stream since they partake in the shift conversion reaction prior to the methanation reaction. It would therefore be desirable to have a method of efficiently and selectively removing carbon dioxide and hydrogen sulfide from coal gas process streams while leaving carbon monoxide, hydrogen and methane in the stream.
Low-Btu coal gas is produced at lower pressure (about 300 psi) as an alternative fuel for combined-cycle power generation. Removal of hydrogen sulfide from such coal gas streams is essential to minimize atmospheric pollution by sulfur dioxide formed during combustion. Bulk removal of carbon dioxide is neither necessary nor desirable, because expansion of this inert gas in the turbine contributes to its power-generating efficiency. Thus, a process capable of selectively removing hydrogen sulfide from such gas streams while leaving carbon dioxide in the stream is required.
It is therefore an object of this invention to provide a novel, inexpensive, and efficient means for the removal of acid gases such as carbon dioxide and hydrogen sulfide from other gases.
It is a further object of this invention to provide a novel and efficient means of selectively removing hydrogen sulfide from a mixture of hydrogen sulfide and carbon dioxide.
It is a further object of this invention to provide a novel and efficient means of selectively separating carbon dioxide from a mixture of carbon dioxide and hydrogen.
It is a further object of this invention to provide a novel and efficient means of selectively separating hydrogen sulfide and/or carbon dioxide from a mixture containing such gases and methane.
It is a still further object of this invention to provide a novel means of cleaning both coal gas and natural gas.
These and other objects are accomplished by the present invention, which is summarized and particularly described below.